U.S. patent application number 11/916451 was filed with the patent office on 2008-09-04 for method and apparatus for monitoring the sedation level of a sedated patient.
This patent application is currently assigned to MED STORM INNOVATION AS. Invention is credited to Hanne Storm.
Application Number | 20080214908 11/916451 |
Document ID | / |
Family ID | 35295070 |
Filed Date | 2008-09-04 |
United States Patent
Application |
20080214908 |
Kind Code |
A1 |
Storm; Hanne |
September 4, 2008 |
Method and Apparatus For Monitoring the Sedation Level of a Sedated
Patient
Abstract
A method and an apparatus for monitoring the sedation level of a
sedated patient during anaesthesia, in particular during a
pre-surgical phase. The method comprises the steps of providing a
skin conductance signal measured at an area of the patient's skin,
calculating a derivative signal of said conductance signal with
respect to time, and establishing said sedation level based on said
derivative signal.
Inventors: |
Storm; Hanne; (Oslo,
NO) |
Correspondence
Address: |
CHRISTIAN D. ABEL
ONSAGERS AS, POSTBOKS 6963 ST. OLAVS PLASS
NORWAY
N-0130
NO
|
Assignee: |
MED STORM INNOVATION AS
Oslo
NO
|
Family ID: |
35295070 |
Appl. No.: |
11/916451 |
Filed: |
June 8, 2006 |
PCT Filed: |
June 8, 2006 |
PCT NO: |
PCT/NO2006/000217 |
371 Date: |
December 4, 2007 |
Current U.S.
Class: |
600/306 |
Current CPC
Class: |
A61B 5/0531 20130101;
A61B 5/16 20130101; A61B 5/4821 20130101; A61B 5/7239 20130101 |
Class at
Publication: |
600/306 |
International
Class: |
A61B 5/00 20060101
A61B005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 10, 2005 |
NO |
20052833 |
Claims
1. Method for monitoring the sedation level of a sedated patient
during anaesthesia, in particular during a pre-surgical phase, the
method comprising the steps of: providing a skin conductance signal
measured at an area of the patient's skin, calculating a derivative
signal of said conductance signal with respect to time,
establishing said sedation level based on said derivative
signal.
2. Method according to claim 1, wherein said step of calculating
said derivative signal comprises to select a first skin conductance
value at the start point of an interval, to select a second skin
conductance value at the end point of said interval, calculating
the derivative signal as the difference between said first and the
second skin conductance values divided by the duration of said
interval.
3. Method according to claim 2, wherein said duration of said
interval is between 10 seconds and 30 seconds, more preferably
between 15 seconds and 25 seconds, and most preferably about 20
seconds.
4. Method according to one of the claims 1-3, wherein said step of
establishing said sedation level comprises applying a non-linear
transformation between said derivative signal and said sedation
level.
5. Method according to claim 4, wherein said transformation is a
discretization function.
6. Method according to claim 4 or 5, wherein said transformation is
arranged for mapping a range of derivative signal values to a
certain level in the OAAS scale.
7. Method according to claim 6, wherein a derivative signal value
in the range [-0.02 .mu.S/s, -0.00 .mu.S/s] corresponds to an OAAS
level of 3 or 2.
8. Method according to one of the claims 1-7, wherein said step of
establishing said sedation level comprises generating an output
signal which indicates said sedation level.
9. Method according to one of the claims 1-8, wherein said step of
providing a skin conductance signal comprises measuring the skin
conductance on the palmar side of the patient's hand.
10. Method according to one of the claims 1-8, wherein said step of
providing a skin conductance signal comprises measuring the skin
conductance on the plantar side of the patient's foot.
11. Method according to one of the claims 1-10, wherein said step
of providing a skin conductance signal comprises measuring the skin
conductance using an alternating current with a frequency in the
range up to 1000 Hz.
12. Method according to one of the claims 1-11, wherein said
patient is a human.
13. Method according to one of the claims 1-11, wherein said
patient is an animal.
14. Apparatus for monitoring the sedation level of a sedated
patient during anaesthesia, in particular during a pre-surgical
phase, the apparatus comprising: measurement equipment for
providing a skin conductance signal measured at an area of the
patient's skin, and a control unit, arranged for performing a
method as set forth in one of the claims 1-13.
15. Apparatus according to claim 14, wherein the measurement
equipment and the control unit comprise features substantially as
disclosed in the above specification.
Description
TECHNICAL FIELD
[0001] The invention relates in general to medical technology, and
in particular to a method and an apparatus for monitoring patients
during surgery and general anaesthesia. More specifically, the
invention relates to a method and an apparatus for monitoring the
sedation level of a sedated patient, in particular during a
pre-surgical phase.
BACKGROUND OF THE INVENTION
[0002] During surgery it is very important to observe the patient's
level of consciousness and awareness. Few reliable methods of
observation exist today. In the field of medical technology there
is a problem in producing physical measurements representing the
activity in an individual's autonomous nervous system, i.e. in the
part of the nervous system, which is beyond the control of the
will.
[0003] Particularly, there is a special need to establish a state
of a sufficiently deep sedation in the patient, in order to avoid
administering more anaesthesia than necessary during surgery and
general anaesthesia.
RELATED BACKGROUND ART
[0004] WO-03/94726 discloses a method and an apparatus for
monitoring the autonomous nervous system of a sedated patient. In
the method, a skin conductance signal is measured at an area of the
patient's skin. Certain characteristics, including the average
value of the skin conductance signal through a time interval and
the number of fluctuation peaks through the interval, is
calculated. Based on these characteristics, two output signals are
established, indicating pain discomfort and awakening in the
patient, respectively. The awakening signal is established based on
the number of fluctuations and the average value through an
interval.
SUMMARY OF THE INVENTION
[0005] An object of the present invention is to provide a method
and an apparatus for monitoring a sedated patient, in particular a
method and an apparatus for establishing a signal indicating the
state of sedation in the patient, based on skin conductance
measurements.
[0006] Another object of the present invention is to provide such a
method and apparatus which provides more reliable output
indications.
[0007] A special object of the invention is to provide such a
method and an apparatus which indicates a state of a sufficiently
deep sedation in the patient, in order to avoid administering more
anaesthesia than necessary during surgery and general
anaesthesia.
[0008] Another object of the present invention is to provide such a
method and apparatus which do not rely on the calculating of the
number of fluctuation peaks or average value of the skin
conductance signal through any measurement interval.
[0009] The above and additional objects are obtained by a method
and an apparatus as set forth in the appended independent
claims.
[0010] Further advantages are achieved by the preferred embodiments
set forth in the dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The invention will now be described in more detail with
reference to the attached drawings, in which
[0012] FIG. 1 is a block diagram for an apparatus according to the
invention, and
[0013] FIG. 2 is a flow chart illustrating a method according to
the invention.
[0014] FIG. 3 is a measurement plot of a time series of an acquired
skin conductance signal.
DETAILED DESCRIPTION OF THE INVENTION
[0015] FIG. 1 illustrates a block diagram for a preferred
embodiment of an apparatus according to the invention. On an area 2
of the skin on a body part 1 of the patient, sensor means 3 are
placed for measuring the skin's conductance. The body part 1 is
preferably a hand or a foot, and the area 2 of the skin on the body
part 1 is preferably the palmar side of the hand (in the palm of
the hand) or the plantar side of the foot (under the sole of the
foot). The sensor means 3 comprise contact electrodes where at
least two electrodes are placed on the skin area 2. In a preferred
embodiment the sensor means 3 consist of three electrodes: a signal
electrode, a measuring electrode and a reference voltage electrode,
which ensures a constant application of voltage over the stratum
corneum (the surface layer of the skin) under the measuring
electrode. The measuring electrode and the signal electrode are
preferably placed on the skin area 2. The reference voltage
electrode may also be placed on the skin area 2, but it is
preferably placed in a nearby location, suitable for the measuring
arrangement concerned.
[0016] In a preferred embodiment an alternating current is used for
measuring the skin's conductance. The alternating current
advantageously has a frequency in the range of up to 1000 Hz,
corresponding to the area where the skin's conductance is
approximately linear. A frequency should be selected which ensures
that the measuring signal is influenced to the least possible
extent by interference from, e.g., the mains frequency. In a
preferred embodiment the frequency is 88 Hz. A signal generator,
operating at the specified frequency, applies a signal current to
the signal electrode.
[0017] In the case of alternating current the conductance is
identical to the real part of the complex admittance, and therefore
not necessarily identical with the inverse value of the resistance.
An advantage of using alternating current instead of direct current
in conductance measurement is that by this means one avoids the
invidious effect on the measurements of the skin's electrical
polarizing properties.
[0018] The resulting current through the measuring electrode is
conveyed to a measurement converter 4. This comprises a current to
voltage converter, which in a preferred embodiment is a
transresistance amplifier, but in its simplest form may be a
resistance, which converts the current from the measuring electrode
to a voltage.
[0019] The measurement converter further comprises a decomposition
circuit, preferably in the form of a synchronous rectifier, which
decomposes the complex admittance in a real part (the conductance)
and an imaginary part (the susceptance). However, it is sufficient
if the decomposition circuit only comprises means for deriving the
conductance. The synchronous rectifier multiplies the measured
voltage with the voltage from the signal generator. The two signals
are in-phase. After multiplication, the result is according to the
cosine (2u) equation, where the result is a DC component and one
component at 2u frequency. In the preferred embodiment, this
becomes 176 Hz. In the preferred embodiment, this synchronous
rectifier is realized as an analog circuit with the required
accuracy.
[0020] The measurement converter 4 may also comprise amplifier and
filter circuits. In the preferred embodiment the measurement
converter contains low-pass filters, both at the input and at the
output. The object of the input low-pass filter is to attenuate
high-frequency noise, for instance coming from other medical
equipments, and also to serve as anti-aliasing filter to prevent
high frequency components from being received by subsequent
circuits for time discretization. The output low-pass filter shall
attenuate the 2u components that result from the multiplication
operation in the synchronous rectifier so that only the signal near
DC is used for further processing.
[0021] By means of the choice of components and design details,
moreover, the measurement converter is designed with a view to
obtaining high sensitivity and a low noise level.
[0022] The control unit 5 comprises a time discretization unit 51
for time discretization of the signal from the measurement
converter. The time discretization takes place at a sampling rate,
which may advantageously be in the order of 20 to 200 samplings per
second. The control unit further comprises an analog-digital
converter 52, which converts measurement data to digital form. The
choice of circuits for time discretization and analog-digital
conversion implies technical decisions suitable for a person
skilled in the art. In the preferred embodiment, time
discretization is done in an integrated circuit, which combines
oversampling, filtering and discretization.
[0023] The control unit may advantageously comprise additional
analog and possibly also digital inputs (not illustrated), in
addition to the input from the measurement converter 4. In this
case the control unit 5 can either be equipped with a plurality of
analog-digital converters 52, or it can employ various multiplexing
techniques well-known to those skilled in the art in order to
increase the number of analog inputs. These additional analog
inputs may, for example, be arranged for additional electrodermal
measurements, or for other physiological measurements which may
advantageously be performed simultaneously or parallel with the
electrodermal measurement, such as temperature, pulse, ECG,
respiratory measurements, oxygen saturation measurements in the
blood, or EEG (bispectral index).
[0024] The measurement converter 4 preferably includes a
synchronous rectifier and a low pass filter; which converts the
measured signal into a voltage. This voltage is further sent to
control unit 5; which includes time discretization module 51 and
analog-digital converter 52, which converts measurement data to
digital form. The choice of circuits for time discretization and
analog-digital conversion implies technical decisions suitable for
a person skilled in the art. In the preferred embodiment, time
discretization is done in an integrated circuit, which combines
oversampling, filtering and discretization.
[0025] The control unit 5 also comprises a processing unit 53 for
processing the digitized measurement data, storage means in the
form of at least one store for storing data and programs,
illustrated as a non-volatile memory 54 and a random access memory
55. The control unit 5 further comprises an interface circuit 61,
which provides an output signal 71. An auxiliary output signal 72
is also shown. Preferably, the control unit 5 further comprises a
further interface circuit 81, which is further connected to display
unit 8. The control unit 5 may also advantageously comprise a
communication port 56 for digital communication with an external
unit, such as a personal computer 10. Such communication is
well-suited for loading or altering the program which is kept
stored in the memory 54, 55 in the control unit, or for adding or
altering other data which are kept stored in the memory 54, 55 in
the control unit. Such communication is also well suited for
read-out of data from the memory 54, 55 in the apparatus, thus
enabling them to be transferred to the external computer 10 for
further, subsequent analysis or storage. A communication port 56 in
the control unit will be advantageously designed in accordance with
requirements for equipment safety for patients, as described in
more detail below.
[0026] In a preferred embodiment the non-volatile memory 54
comprises a read-only storage in the form of programmable ROM
circuits, containing at least a program code and permanent data,
and the random access memory 55 comprises a read and write storage
in the form of RAM circuits, for storage of measurement data and
other provisional data.
[0027] The control unit 5 also comprises an oscillator (not shown),
which delivers a clock signal for controlling the processing unit
53. The processing unit 53 also contains timing means (not shown)
in order to provide an expression of the current time, for use in
the analysis of the measurements. Such timing means are well-known
to those skilled in the art, and are often included in micro
controllers or processor systems which the skilled person will find
suitable for use with the present invention.
[0028] The control unit 5 may be realized as a microprocessor-based
unit with connected input, output, memory and other peripheral
circuits, or it may be realized as a micro controller unit where
some or all of the connected circuits are integrated. The time
discretization unit 51 and/or analog-digital converter 52 may also
be included in such a unit. The choice of a suitable form of
control unit 5 involves decisions, which are suitable for a person
skilled in the art.
[0029] An alternative solution is to realize the control unit as a
digital signal processor (DSP).
[0030] Several structural hardware components of the present
invention may be identical to those used in WO-03/94726. However,
the method or process performed by the control unit 5, in order to
analyze the skin conductance signal, is distinctive and
substantially different from the method/process disclosed in
WO-03/94726.
[0031] The data processing unit 53 is arranged for analysing the
measured and digitized signal provided by the A/D converter 52. The
signal is analysed in order to extract different types of
information.
[0032] The control unit 5 is arranged to read time-discrete and
quantized measurements for the skin conductance from the
measurement converter 4, preferably by means of an executable
program code, which is stored in the non-volatile memory 54 and
which is executed by the processing unit 53. It is further arranged
to enable measurements to be stored in the read and write memory
55. By means of the program code, the control unit 5 is further
arranged to analyze the measurements in real time, i.e.
simultaneously or parallel with the performance of the
measurements.
[0033] In this context, simultaneously or parallel should be
understood to mean simultaneously or parallel for practical
purposes, viewed in connection with the time constants which are in
the nature of the measurements. This means that input, storage and
analysis can be undertaken in separate time intervals, but in this
case these time intervals, and the time between them, are so short
that the individual actions appear to occur concurrently.
[0034] The processing unit 53, the memories 54, 55, the
analog/digital converter 52, the communication port 56, the
interface circuit 81 and the interface circuit 61 are all connected
to a bus unit 59. The detailed construction of such bus
architecture for the design of a microprocessor-based instrument is
regarded as well-known for a person skilled in the art.
[0035] The interface circuit 61 is a digital port circuit, which
derives output signals 71, 72 from the processing unit 53 via the
bus unit 59 when the interface circuit 61 is addressed by the
program code executed by the processing unit 53.
[0036] The output signal 71 indicates the sedation level in the
patient. Preferably, the output signal 71 indicates that the
analysis of the skin conductance measurement has detected that the
patient has reached a sufficiently deep sedation level.
[0037] In a preferred embodiment the display means 8 consists of a
screen for graphic visualization of the conductance signal, and a
digital display for displaying the frequency and amplitude of the
measured signal fluctuations. The display units are preferably of a
type whose power consumption is low, such as an LCD screen and LCD
display. The display means may be separate or integrated in one and
the same unit.
[0038] The apparatus further comprises a power supply unit 9 for
supplying operating power to the various parts of the apparatus.
The power supply may be a battery or a mains supply of a known
type.
[0039] The apparatus may advantageously be adapted to suit the
requirements regarding hospital equipment, which ensures patient
safety. Such safety requirements are relatively easy to fulfill if
the apparatus is battery-operated. If, on the other hand, the
apparatus is mains operated, the power supply shall meet special
requirements, or requirements are made regarding a galvanic
partition between parts of the apparatus (for example, battery
operated), which are safe for the patient and parts of the
apparatus, which are unsafe for the patient. If the apparatus has
to be connected to external equipment, which is mains operated and
unsafe for the patient, the connection between the apparatus, which
is safe for the patient and the unsafe external equipment requires
to be galvanically separated. Galvanic separation of this kind can
advantageously be achieved by means of an optical partition. Safety
requirements for equipment close to the patient and solutions for
fulfilling such requirements in an apparatus like that in the
present invention are well-known to those skilled in the art.
[0040] FIG. 2 illustrates a flow chart for a method for monitoring
the sedation level of a sedated patient. The method is particularly
used for monitoring the sedation level of the sedated patient
during anaesthesia, and specifically, for monitoring the sedation
level during a pre-surgical phase of the anaesthesia period.
[0041] The method starts at reference 31.
[0042] Next, in the acquiring step 32, a skin conductance signal or
EDR (electrodermal response) signal is measured at the area 2 of
the patient's skin, time-quantized and converted to digital form
using the equipment described with reference to FIG. 1. A
time-series of a certain duration, typically a period of at least
20 seconds, containing skin conductance data, is acquired during
this step. With a sampling rate of 20-200 samples per second, the
time-series may contain 400-4000 samples, respectively. The data is
stored in a portion of the memory 55.
[0043] Advantageously, the acquiring step 32 also comprises a
prefiltering process, wherein the measured data are filtered in
order to remove high frequency noise, and irrelevant anomalies such
as peaks or spikes caused by interference or measurement
errors.
[0044] Next, in the derivative calculation step 33, a derivative
signal of said conductance signal with respect to time is
calculated by the processing unit 53. The calculating step
advantageously comprises the following substeps:
[0045] First, to select a first skin conductance value at the start
point of an interval, next, to select a second skin conductance
value at the end point of the interval, and then calculating the
derivative signal as the difference between said first and the
second skin conductance values divided by the duration of the
interval.
[0046] The duration of the interval is advantageously between 10
seconds and 30 seconds. More preferably the interval is between 15
seconds and 25 seconds. Most preferably the interval is about 20
seconds.
[0047] The calculated derivative signal is also stored in a portion
of the memory 55.
[0048] Next, in step 34, a sedation level is established based on
the derivative signal calculated in step 33.
[0049] Advantageously, the establishing step 34 comprises applying
a non-linear transformation between the derivative signal
calculated in step 33 the output sedation level. The transformation
is preferably a discretization function, wherein said
transformation is arranged for mapping a range of derivative signal
values to a certain level in the OAAS scale.
[0050] The transformation is advantageously implemented as a series
of comparison processing steps operating in accordance with limit
values stored in a table in a portion of the memory 54.
[0051] The Observer's assessment of anaesthesia and sedation (OAAS)
scale is well known in the art, used for assessing the hypnotic
state of patients. The levels are given below:
[0052] Level 5: Patient is awake, eyes are open, and patient
replies and responses readily to spoken commands.
[0053] Level 4: Patient is sedated, he or she responses to spoken
commands such as `squeeze my hand`, but has mild ptosis,
drowsiness.
[0054] Level 3: Patient responses only to loud spoken commands. The
eyelid reflex is still present.
[0055] Level 2: Patient does not response to spoken commands. The
eyelid reflex is not present.
[0056] Level 1: Patient does not response with movement to TOF
stimulation (50 mA). No muscle relaxants are assumed.
[0057] Level 0: Patient does not response with movement to tetanic
(50 mA) stimulation of the ulnar nerve. No muscle relaxants are
assumed.
[0058] The establishing step 34 advantageously comprises to
determine if the derivative signal value is in the range [-0.04
.mu.S/s, -0.00 .mu.S/s], and if so, to set the sedation level to an
OAAS level of 4 or less.
[0059] The establishing step 34 advantageously comprises to
determine if the derivative signal value is in the range [-0.04
.mu.S/s, -0.02 .mu.S/s], and if so, to set the sedation level to an
OAAS level of 4 or 3.
[0060] The establishing step 34 advantageously comprises to
determine if the derivative signal value is in the range [-0.02
.mu.S/s, -0.00 .mu.S/s], and if so, to set the sedation level to an
OAAS level of 3 or 2.
[0061] The establishing step 34 advantageously comprises to
determine if the derivative signal value is substantially 0.02
.mu.S/s, and if so, to set the sedation level to an OAAS level of 2
or less.
[0062] Advantageous characteristics of the function between the
derivative signal and the sedation level are given in table 1
below:
TABLE-US-00001 TABLE 1 Sedation level vs. derivative SC signal
Derivative of SC signal, range Discretizated sedation level output
(.mu.S/s) (OAAS level) -0.03 +/- 0.01 OAAS 4-3 -0.01 +/- 0.01 OAAS
3-2 0 OAAS <2
[0063] Advantageously, the step 34 of establishing said sedation
level further comprises generating an output signal which indicates
said sedation level. The signal may be a digital signal which is
displayed on the display 8, and/or output as the output signal 71
to an external equipment.
[0064] The invention thus relates both to the method described with
reference to FIG. 2, and to an apparatus for monitoring the
sedation level of a sedated patient during anaesthesia, in
particular during a pre-surgical phase. Structurally, the apparatus
may be as substantially described with reference to FIG. 1. The
apparatus comprises measurement equipment for providing a skin
conductance signal measured at an area of the patient's skin, and a
control unit which is arranged for performing a method
substantially corresponding to the method illustrated in FIG.
2.
[0065] After the completion of the establishing step 34, the
process may be terminated as illustrated by the terminating step
35. Alternatively, the process may be repeated, using another
measurement acquiring period and/or another period for calculating
the derivative signal.
[0066] FIG. 3 is a measurement plot of a time series of an acquired
skin conductance signal during a pre-surgical anaesthesia phase.
The signal is measured in the palm of a human patient, in the
period of 0 to 300 seconds after propofol infusion.
[0067] As shown in FIG. 3, the SC signal decreases with time, i.e.
as the sedation level of the patient changes from awake (OAAS level
5 at 0 seconds) to deeper sedation (at about 300 seconds). The
derivative of the signal is calculated as the relative difference
(i.e. divided by the duration of the interval) between the start SC
value and the end SC value over a period of typically 20 seconds.
As will be appreciated, the derivative signal is negative in most
of the pre-surgical anaesthesia phase, indicating that the sedation
level increases (i.e., the OAAS level is reduced). Moreover an
increase in the derivate is expected if the OAAS level is
increased.
[0068] The skilled person will realize that the above description
has been presented as an detailed example of a particular
embodiment, and that the principles of the invention may be put
into effect in other ways as well. As an example, the skilled
person will realize that skin resistance may be measured instead of
skin conductance, provided that the inverse nature of these
variables is taken into account.
[0069] When the term "patient" is used throughout the specification
and claims, is should be appreciated that although the present
invention is primarily directed towards the monitoring of human
beings, the invention has also been proven to be applicable for
monitoring animals, in particular mammals. Consequently, the term
"patient" should be interpreted as covering both human and animal
patients.
* * * * *